Atomic Weight of Sulphur

That the atomic weight of sulphur is approximately 32 and not a multiple or sub-multiple of this figure is evident from the following considerations:

The specific heat of rhombic sulphur between 23° and 92° C. is 0-1759. Assuming a mean atomic heat of 6.4, by Dulong and Petit's Law the atomic weight is approximately 36.

Experiment shows that, if Avogadro's hypothesis be accepted, sulphur is never present in its volatile compounds in an amount less than 32 grams per gram-molecule.

Sulphur, with an atomic weight of 32, fits naturally into the periodic scheme.

The atomic number of sulphur (16) places the element between phosphorus (15) and chlorine (17), so that its atomic weight should lie between 31 and 35.5.

The mass spectrum shows that three isotopes of sulphur exist, namely 32, 33 and 34. S34 is about three times as abundant as S33, and the two together probably amount to about 3 per cent, of the whole.

The exact atomic weight of sulphur has been determined in two different ways, namely:

By gasometric methods.

By gravimetric methods.

Gasometric Methods

Only the most recent of these are of sufficient accuracy to lend themselves for useful atomic weight determination, and even then, owing to the uncertainty of the corrections to be applied in view of the fact that the gases concerned do not strictly obey the gas laws, the results cannot be regarded as equal in accuracy to those obtained by gravimetric methods. The best result would appear to be that calculated from the density of sulphur dioxide and its compressibility, as determined by Jaquerod and Scheuer, namely S = 32.059.

Gravimetric Methods

The early determinations, down to the time of Stas, are now of historical interest only. They are therefore included without further comment in the accompanying table. In the first series of Stas' experiments a known weight of silver was heated in the vapour of sulphur or in a current of pure hydrogen sulphide. Excess sulphur was removed by distillation in a current of carbon dioxide, the latter having been purified with exceptional care, as traces of impurity in the gas might cause grave error. As a mean of five results, calculated to vacuum, Stas found:

2Ag:Ag2S = 100:114.8522±0.0007,

whence the atomic weight of sulphur is 32.045.

By reducing silver sulphate in a current of hydrogen Stas also determined the ratio:

Ag2SO2:2Ag = 100:69.203,

as a mean of six results, whence the atomic weight of sulphur is 32.022.

These values are too low.

Turning now to more recent work, mention may be made of that of Richards. Three of the eight determinations of the ratio Na2CO3:Na2SO4 made by Richards in 1891 were somewhat doubtful. Omitting these, the mean ratio for the remaining five experiments becomes:

Na2CO3:Na2SO4 = 100:133.9952,

whence S = 32.032. This value, however, is still slightly too low. Redetermination of the same ratio by Richards and Hoover in 1915 gave S = 32.054, and this agrees with the value S = 32.059 determined by volumetric methods.

It is difficult to understand why the foregoing values should still be so much lower than that determined by Richards in 1907 from the ratio 2AgCl:Ag2SO4, namely 32.068, for this latter value agrees closely with the 32.065 derived by Burt and Usher in 1911 from the ratio 2N2:N4S4. The value accepted by the "International Atomic Weights Commission" in 1925 was 32.064, whilst the Revised Table for 1929 published by the Council of the Chemical Society gives the value 32.06(5), in which the last figure may be in error by two or three units.